1. Field of the Invention
The present invention relates to a porous molded article comprising a stretched film having continuous or closed microvoids therein. More specifically, the present invention relates to a porous molded article composed of a crystalline olefinic resin matrix and a cyclic olefin polymer island dispersed therein.
The porous molded article has excellent printability and is useful as synthetic paper and packing material.
2. Description of the Background
Hitherto, a porous molded article made up of a crystalline olefinic resin and having microvoids has been widely used as fine porous films and hollow fibers for use as filters; liquid permeable films and diaphragms for use as separators; gas-permeable films for use as packing materials; moisture-permeable waterproof films for use as clothes; synthetic papers for printing; etc.
These porous molded articles having microvoids made up of a crystalline olefinic resin have been produced by the following methods.
1) A film made up of a blend of a crystalline olefinic resin and fine inorganic particles is stretched whereby microvoids are formed in the interface between the crystalline olefinic resin and each of the fine inorganic particles as described in Kobunshi Ronbun Shu (Collected High Molecular Theses), Vol. 48, No. 8, 463(1991); ibid., Vol. 48, No. 8, 491(1991), JP-B-46-40794 (the term "JP-B" as used herein means an "examined published Japanese patent application), etc. PA1 2) A film made up of a blend of a crystalline olefinic resin and a polymer component which is incompatible with the crystalline olefinic resin, such as polyamide, polycarbonate, polybutylene terephthalate, or the like, is stretched, which results in the formation of microvoids at the interface between the crystalline olefinic resin and the polymer component as described in JP-A-59-209849 (the term "JP-A" as used herein means an "unexamined published Japanese patent application", JP-B-3-24334), etc. PA1 3) A film of a crystalline olefinic resin to which has been previously added a specific large amount of an additive such as plasticizer, or the like, is prepared, and then the additives are extracted using an organic solvent, water, etc., to form continuous holes in the film as described in JP-A-60-242035, JP-A-7-309965, etc.
However, the conventional methods have the following problems.
That is, in method 1) of stretching the film made up of the blend of a crystalline olefinic resin and fine inorganic particles which form microvoids at the interface between the crystalline olefinic resin and each of the inorganic particles, when the fine inorganic particles are blended with the crystalline olefinic resin, thereby resulting in many microvoids or the formation of continuous fine holes, it is required to blend a large amount of the fine inorganic particles. Accordingly, the specific gravity of the obtained film becomes large and secondary aggregation of the fine inorganic particles occurs easily. It makes the diameters of the microvoids formed in the film not only nonuniform, but also the film fractures when stretched because the aggregated particles act as the fracture points, which make it difficult to achieve stretching at a high ratio, whereby it is very difficult to form a thin film. Also, there are problems that surface roughness on the surface of the stretched film occurs and a part of the largely blended fine inorganic particles falls from the surface to make the processing machine dusty.
In the case of method 2), because the dispersibility of the polymer component which is incompatible with the crystalline olefinic resin is generally poor, when using a large amount of the polymer component, the islands of the polymer dispersed in the crystalline olefinic resin matrix become coarse particles, and microvoids cannot be formed in the film, and also the stretching temperature range is narrowed. Also, if a lesser amount of the polymer component incompatible with the crystalline olefinic resin is used, the light transmittance of the stretched film is high and an opaque stretched film is not obtained.
In method 3) of previously adding a specifically large amount of an additive such as a plasticizer, etc., to a crystalline olefinic resin and during or after the formation of film, extracting the additive using a solvent to form continuous holes, a large amount of the additive and the extraction step are required. In this method, the continuous holes are the result of the extraction, that is, the continuous holes are not formed if the extraction step is not applied.
Also, in method 3), in order to obtain the continuous holes in a high amount, an amount of additive, 2 to 3 times or even more of the amount of the crystalline olefinic resin is required. Thus, in method 3), the operation is very complicated and also production costs become high, which make mass production difficult.
As described above, in the above-described conventional techniques of producing porous molded articles, problems are encountered which make it very difficult to prepare a porous molded article of a thin film having many microvoids by stretching at a high ratio. That is, a production method requiring a complicated operation must be used.
As a means of solving these problems, JP-A-8-27294 provides a resin-stretched film made of, as the base material, a thermoplastic resin containing (A) from 50 to 95% by weight a propylene series resin and (B) from 5 to 50% by weight a cyclic olefinic resin selected from an open-ring polymer or an open-ring copolymer induced from a cyclic olefin, the hydrogenated product of said polymer or copolymer, and an addition polymer of a cyclic olefin and represented by formula (1) and ethylene, said cyclic olefinic resin having a limiting viscosity .eta.! measured in decalin at 135.degree. C. of from 0.05 to 10 dl/g and a softening temperature of from 145 to 170.degree.; ##STR1## wherein m represents 0 or a positive integer and R.sup.1' to R.sup.12' each independently represents a hydrogen atom, a halogen atom, or a hydrocarbon group; said R.sup.9' to R.sup.12' may combine with each other to form a monocyclic or polycyclic group; and said R.sup.9' and R.sup.10' or said R.sup.11' and R.sup.12' may together form an alkylidene group, said opaque resin stretched film have fine holes wherein the voids of the resin stretched film is in the range of from 20 to 70% and the film having an opacity of at least 75%.
However, when print is applied onto the opaque, stretched resin film with an offset ink which is used for quick drying offset printing, the olefinic resin of the film matrix is swelled with the vehicle in the ink which results in partial unevenness on the surface of the printed stretched film or when printing is applied onto the whole surface of the stretched film, the whole stretched film is curled. That is, when printing is applied to the opaque, stretched resin film, a so-called solvent attack occurs which makes it difficult to practically use the opaque, stretched olefinic resin film.
The composition of a drying type offset ink widely used is shown in Table 1 and Table 2.
TABLE 1 ______________________________________ Fundamental Composition of Offset Ink ______________________________________ 1 #STR2## ______________________________________
TABLE 2 ______________________________________ Composition of Offset Printing Ink (weight part) Specific Offset Widely-Use Printing Ink Offset For Non-absorptive Printing Ink Material ______________________________________ Pigment 15 to 50 (30) (25) Resin 25 to 30 (20) (30) Dry oil 10 to 15 (10) (34) H. B. Pet. Solv.* 20 to 45 (35) (5) Dryer 0.5 to 2 (1) (2) Others 2 to 5 (4) (4) (100) (100) ______________________________________ (*): HighBoling Petroleum Solvent
Accordingly, a specific offset ink formulated at a sacrifice to its drying property is practically used as the offset printing ink for olefinic resin films. However, because in the case of the specific offset ink for the olefinic resin films, the drying time is long and the printing shops and the ink makers are limited to specific formulations, the development of an olefinic resin film for which a wide-use offset ink can be used has been desired.